As to imaging apparatuses such as digital still cameras (hereinafter, referred to as digital cameras), it is important to increase the battery life. Consequently, methods to achieve power saving in the control of a digital camera are conventionally used. As one of them, there is a technology to try power saving by lowering a frame rate during monitoring.
The frame rate can be lowered by lowering a frequency of a clock to be supplied to an image pickup device, a timing generator (hereinafter, referred to as TG), an A/D converter, and the like. Since a rate of output of data from an image sensor is lowered by lowering the frame rate, power consumption per unit time in a signal processing IC can be reduced.
As such a technology, for example, as shown in Japanese Patent Laid-Open Publication Number 2007-104564, there is disclosed an imaging apparatus which maintains response in photographing and also achieves low power consumption in a low power consumption mode that stops a screen display; which reduces power consumption of an image pickup device by driving a CCD by a slow clock; which provides a quick response by performing photometry continuously; and which reduces power consumption by stopping an image processing in an image processing unit and an operation in a display device.
Further, as shown in Japanese Patent Laid-Open Publication Number 2003-92698, there is disclosed an imaging apparatus which can change a frequency of a clock which is output from a clock generator to a timing generator in accordance with an updating speed of a frame, set by a user, during monitoring.
Furthermore, as shown in Japanese Patent Laid-Open Publication Number 2003-60994, there is disclosed an imaging apparatus of which a driving frequency to drive a CCD can be changed over between a first frequency and a second frequency, which is lower than the first frequency, that are generated from a clock supplied from a clock generation device in accordance with a plurality of predetermined read-out modes. In a technique described in Japanese Patent Laid-Open Publication Number 2003-60994, a balance between power saving and performance is achieved by changing the driving frequency in accordance with a driving mode (a read-out mode) of an image pickup device, for example, by heightening the frequency in a mode such as for a still image loading which has to read out a large number of pixels.
Moreover, as shown in Japanese Patent Laid-Open Publication Number 2008-35474, there is disclosed a digital camera which performs displaying of a through-image at a first updating cycle until a half-pressing operation of a release button and at a second updating cycle, which is shorter than the first updating cycle, after the half-pressing operation of the release button.
However, according to the above-mentioned techniques described in Japanese Patent Laid-Open Publication Numbers 2007-104564, 2003-92698, and 2003-60994, while power saving can be achieved by lowering a frame rate for monitoring, if the frame rate is lowered too much, a problem arises, for example, such that a subject following capability worsens and a finder function as a camera cannot be performed. Further, a problem arises such that the display quality level decreases by an increase in the amount of smear in a case where a CCD is adopted as an image pickup device. Furthermore, there is a problem such that a control response worsens. That is to say, a control timing depends on the frame rate since an image is generally processed in synchronization with each frame in a signal processing IC.
On the other hand, according to the technique described in Japanese Patent Laid-Open Publication Number 2008-35474 mentioned above, regarding the problem such that a finder function cannot be performed, it is easy to get a timing to release a shutter since a through-image displayed on a monitor becomes smooth by heightening a frame rate after the half-pressing operation of the release button. In addition, regarding the problem such that a control response worsens, the response does not worsen since still image photographing is started from a monitoring state at a high frame rate by heightening the frame rate after the half-pressing operation of the release button. However, in a technique described in Japanese Patent Laid-Open Publication Number 2008-35474, power saving cannot be achieved after the half-pressing operation of the release button, and nothing is disclosed regarding shifting from a monitoring state at a low frame rate to still image photographing.
In addition, in a photographing sequence, a state of a signal processing IC when shifting from a monitoring state to still image photographing is shifted in the following order: “monitoring data loading”, “monitoring image loading stop”, and “still image data loading”. Accordingly, a processing time for performing stopping of loading after the monitoring data loading depends on a frame rate for monitoring. That is to say, a lot of time is needed to stop loading if the frame rate is lowered for power saving; therefore, there is a problem such that a release time lag is increased. Further, there is a problem such that a lot of time is needed for a monitoring data loading stop processing not only in a case of shifting from a monitoring state at a low frame rate to still image photographing, but also in a case of shifting from a monitoring state at a low frame rate to another state.